Combined volume and tone control



March 12, 1935. w. VAN B. ROBERTS COMBINED VOLUME AND TONE CONTROL Filed April 22, 1930 INVENTOR WALTER VAN B. ROBERTS ll lvinkllllkl ATTORN EY Patented Mar. 12, 1935 UNITED STATES COMBINED VOLUME Walter van B. Roberts,

AND TONE CONTROL Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application April 22, 1930, Serial No; 446,202

12 Claims.

My present invention relates to acoustic reproducing devices, and more particularly to combined volume and tone control systems adapted for use with audio frequency currents.

In connection with radiotelephone receiving sets, electromagnetic pick-ups or other phonograph devices, public address systems, and audio output circuits of sound moving picture systems, fidelity of reproduction with respect to the tone of the music, or speech, being reproduced, is desired. Extensive research has revealed that due to the nature of the human ear, music, or speech, reproduced at a high level of intensity should have a lesser proportion of low frequency amplitude, as compared with high frequency, than if the music, or speech, were reproduced at a low level, if apparent faithfulness of reproduction is to be preserved.

In my copending application Serial No. 445,937

filed April 21, 1930, there has been disclosed a method of, and arrangements for, controlling the tone of electrically reproduced sounds, which method utilizes a plurality of amplifying, or filtering, devices possessing difierent frequency characteristics, a controlling device for proportioning in opposite sense the input to the first devices in accordance with the general, or average, intensity of the input whereby the inputs of each of the amplifiers are relatively adjusted in a predetermined, and automatic, manner as the audio input varies. Additionally, it has been proposed of late to amplify audio currents resulting from sources as aforementioned, in such a manner that.

the amplification increases with an increase of average signal strength, but decreases when the input signal strength becomes abnormally low. This results in a greater contrast between the maximum and minimum loudness during the rendition of a sound passage, as music. Such contrast is desirable during reproduction to restore the sound passage to its original condition, inasmuch as an inverse process is usually employed in recording or transmitting the sound passage to be-reproduced. That is to say, during rendition, or transmission, the vocal passage is not amplified as much while it is loud, as it is when very soft.

Now, I have discovered that the aforementioned two control processes, viz., automatic tone con trol as disclosed in my aforementioned copending application, and the above method of differential intensity control, may be combined into a single control method, whereby the benefits of both control systems will be secured during the reproduction of a particular sound passage, this new control process while resulting in a greater contrast between the maximum and minimum loudness during'the reproduction of the sound passage, at the same time preserving fidelity of reproduction by properly proportioning the low and high audio frequencies during the variations of volume.

Accordingly, it is one of the main objects of my present invention to provide a method of, and means for, expanding the range of volumes in reproducing sound passages, and simultaneously varying the ratio of low to high audio frequency output in accordance with the general 'level of volume.

Another important object of the invention is to provide a plurality of selective amplifiers, each of which is arranged to stress a predetermined independent portion of the audible spectrum, means for automatically regulating, in a predetermined manner and in accordance with the input to the said amplifiers, the tone of the audio output of said amplifiers, and additional means, operative simultaneously with said tone regulating means, for increasing total amplification in both amplifiers with an increase of average audio intensity, but decreasing the total amplification of the amplifiers when the input audio intensity becqmes abnormally low.

Another object of the invention is to provide a unitary control arrangement for audio amplifying systems which includes means for automatically regulating the tone of the acoustic output of the reproducers connected to the amplifying systems, said regulating means, additionally, functioning to produce a greater contrast between the maximum and minimum loudness during the reproduction of a sound passage.

Still another object of the invention is to provide a method of audio amplification which consists in supplying signal energy to a plurality of transducers, said transducers having inverse frequency characteristics, the total gain of the combination of transducers increasing with increases of signal input while the gain at high frequency relative to that at low frequencies of the 4 entire system also increases with increasing average signal input, and simultaneously controlling the tone output of the transducers by means actuated in accordance with the average signal 0 input intensity.

Another object of the invention is to provide an electro-acoustic system having a plurality of reproducers, the tones and volumes of which may be varied simultaneously and automatically in accordance with the acoustical conditions within the range of the reproducers.

Still other objects of the invention are to improve generally the efflciency of audio amplifier systems, and to provide a combined volume and tone control system for audio amplifiers which is not only reliable in operation but automatic in regulation, and economical in circuit arrangement.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit arrangement whereby my invention may be carried into effect.

Referring to the accompanying drawing in which is shown diagrammatically a circuit embodying my invention, there is shown an audio amplifier circuit the input leads 1 of which, have impressed across them modulated audio input energy. The input energy may be derived from any desired source such as the output circuit of the detector stage of a radio telephone receiving set, the output terminals of an electromagnetic pick-up device or other electric phonograph device, the output terminals of the microphone arrangement of a public address system, and from the output terminals of the photo-electric device of a sound motion picture arrangement, or any other source of modulated audio frequency currents. I have not shown any of these sources in the drawing connected to leads 1, for the reason that they are well known to those skilled in the art, and to preserve simplicity of description throughout this specification.

The audio frequency input, applied to the input leads 1, is divided into three paths. One of these paths, by means of leads 50, conducts the input current into an audio amplifier A; a second path, by means of leads 51, conducts the current into an audio amplifier B; while the third path, by means of leads 52, conducts the audio input current into a regulating auxiliary or pilot circuit C. It will be recognized that these three .essential elements of the invention are the same as in the case of the circuit shown in Fig. 2 of my aforementioned copending application.

The regulating circuit C consists of one or more stages of audio amplification and a rectifier stage, the regulating circuit shown in the drawing specifically including in the input circuit of the first audio stage a triode 35, the control electrode of which triode is negatively biased from a source C. An audio transformer T couples the input circuit of the triode to the leads 1, the latter being connected by the leads 52 to the primary coil 3'7 of the transformer T. The anode of the triode 35 has potential applied to it from a source B, through a resistance 38, the source B, if desired, being the same source which supplies potential to the anode of the triode 39 of the second audio stage, through a second resistance 40.

The control electrode of the triode 39 and the anode of triode 35 are connected through a capacity 41, the control electrode and cathode of the triode 39 having connected in series between them, a resistance 42 and a source of negative bias 42. A third triode 43 is connected to the output circuit of triode 39, a capacity 44 connecting the anode of triode 39 and control electrode of triode 43. The control electrode and cathode of triode 43 have connected in series between them, a resistance 45 and a source of biasing potential 45'.

It will thus be seen that there is connected to the source of audio frequency current, the input circuit of a pilot or controlling circuit C, which circuit consists of one or more stages of audio amplification, which may be of any desired type, feeding a rectifier, such as an electron discharge tube of the triode type. The control electrode of the rectifier tube is provided with sufficient bias to reduce the plate current of the tube to a small value in the absence of impressed voltages upon its control electrode. The triode 43 acts as the rectifier, the biasing source 45 biasing the control electrode of triode 43, to sufiiciently reduce the anode current of the triode to a small value, in the absence of amplified voltages from the output circuit of the triode 39, which amplified voltages are to be impressed upon the control electrode of the triode 43. The rectified output of the triode 43 is smoothed out by the filter arrangement, the latter being designed in such a manner that the current output of the filter arrangement varies only slightly with variations in audio input strength.

The output of the pilot amplifier, rectifier and filter, is impressed across the potentiometer 53, the low potential terminal of the resistor of the po-. tentiometer being connected by a lead 54 to the positive terminal of a source of potential 55, the negative terminal of which source is connected to one of the cathode leads of the amplifier A. The screen element 57, of an electron discharge tube 58, which tube is included in the first stage of the amplifier A, is adjustably connected by a lead 56 to the high potential end of the potentiometer resistor, it being noted that the screen element is connected to ground through a capacity 59. The electron discharge tube 58 is preferably a screen grid tube, whereby a stage of screen grid amplification is provided in the amplifier A, the control electrode of the tetrode 58 being negatively biased by a source C1.

The anode of the tetrode 58 has potential applied to it from source B, through a resistance 60, the anode being "connected to the control electrode of a triode 61, through a capacity 62. The control electrode and cathode of the triode 61 have connected in series between them, a choke coil 62 and a source of biasing potential C2, the anode of the triode 61 receiving potential from a source B1. The output of the triode 61 is connected to the input of a reproducing unit 63, the reproducing unit being connected to a common loud speaker S.

The screen element 57, of a screen grid electron discharge tube 58 which is included in the first stage of the amplifier B, is adjustably connected by a lead 56 to a point on the resistor of the potentiometer which is at a lower potential than the point to which the lead 56 is connected, the screen element 57 being connected to ground through a capacity 59. The control electrode of the triode 58' is negatively biased by the source C1, through the lead 51, and the anode of the tetrode has potential applied to it'from a source B2, through a resistance 70. I

The anode of the tetrode 58' is connected to the control electrode of the output triode 72 by capacity '73, the control electrode and cathode of the triode '72 having connected between them in series, a resistance 74 and a source of biasing potential C4, the resistance '74 being shunted by a capacity '71. The anode of the triode 72 has potential applied to it from a source B3, the output of the triode '72 being impressed upon a second reproducer unit 63, the output of which unit is also connected to the common speaker S.

It will thus be seen that each amplifier A; and B includes a stage of screen grid amplification,

' so designed that the amount of amplification depends upon the potential of the screen in each stage becoming greater as the screen potential is increased. The screen elements of the two tubes 58 and 58 are connected to different points of the potentiometer 53, thus insuring that cur-. rent flowing out of the rectifier will make one screen receive more increase of potential than the other, thus increasing the amplification of both amplifiers, but chiefly the amplification of A.

It is to be noted that amplifier A amplifies high audio frequencies best because'the impedance of the choke coil 62 between grid and cathode is greatest for high frequencies, while the amplifier B amplifies low audio frequencies best because 74. The outputs of the two amplifiers A and B are mixed in a common speaker S, it being clearly understood that as an alternative arrangement for mixing the amplifier output, the outputs of each amplifier A and B can be impressed upon a separate speaker, or the outputs may be mixed electrically by well known means, and the combination of currents act upon a single speaker.

The connections of the leads 54, 56 and 56 are so arranged relative to the potentiometer 53 that an increase in audio input strength results in an increase in amplification of both the high and low frequency amplifiers, but the increase is greater in the high frequency amplifier. Consequently, not only is the total amplification of the system increased with increasing input, but also the ratio of high to low frequency amplification is simultaneously increased.

It will of course be understood that automatic control of the tonal output of the audio amplifier is secured, the automatic arrangement being preadjusted for any audio frequency input intensity average value, the method disclosed consisting in utilizing an audio signal to energize the separate transducers A and B, the transducers having inverse frequency characteristics, and whose outputs are combined so as to be simultaneously effective on the listener, the ratio of inputs to the transducers being controlled by means responsive to the energy level of the signals averaged over a period of time which is long compared to the longest period of audio current, it being particularly pointed out that the filter arrangement in the circuit C is designed so that the current output of the filter varies only slowly with variations in audio input strength.

While I have shown particular means for carrying out the automatic variation of combined tone and volume characteristic of the entire audio system in accordance with input-signal level, I do not mean to imply that the means shown are the only feasible methods for achieving the desired results. For example, the two or more amplifier systems may be caused to have the desired frequency characteristics, not only by the design of the amplifiers themselves but equally well by the insertion of the filters, or other networks in series with the amplifiers or even by choosing loud speakers which have difierent frequency characteristics themselves. Furthermore, it is possible to employ a single amplifier between the combined output of the amplifiers A and B, and the common speaker S that is to say, a common amplifier after the two transducers of diverse characteristics, and before the common speaker S.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be'made without departing from the scope of my invention as set forth in the appended claims.

What I claim is: h

1. In a sound reproducing system, the combination with a source of audio frequency energy of an amplifier, an acoustic reproducer connected therewith, and means for automatically controlling the combined tonal and volume output from said reproducer in accordance with acoustic requirements, said means including a pilot circuit connected between the source and an amplification control element of the amplifier adapted to vary automatically and simultaneously the relative intensities of low and high frequency currents amplified in said amplifier and the total amplification of the amplifier in accordance with the average intensity of the energy level of said source.

2. The method of adapting a sound reproduction to predetermined acoustib requirements which consists in changing electrical oscillations, including a predetermined range of frequencies, to acoustic oscillations, and controlling the acoustic oscillations automatically by regulating the volume of the high frequency sounds and independently varying to a diflerent extent the low frequency sounds in accordance with the average intensity of the electrical oscillations, and simultaneously varying the total amplification of the electrical oscillations in the same direction as said average intensity.

3. An amplifier for electrical impulses comprising in combination, a first unit constructed and arranged to amplify comparatively slightly electricalimpulses having a predetermined band of frequencies, and to amplify comparatively greatly another band of said frequencies, a second unit having frequency-amplification characteristics which are inverse to the first unit, acoustic devices associated with said units, and means for automatically proportioning the ratio of gain of each of the aforesaid units in accordance with the average intensity of the input, and additional means connected between said last means and said units whereby the total amplification of the amplifier is varied with varying input.

4. In a system for reproducing electrical oscillations, a first selective amplification unit arranged to amplify high audio frequencies better than low audio frequencies, a second selective amplification unit having frequency characteristics inverse to that of the first unit, and a common means for automatically varying the amount of gain in the first and second units in such a manner that increase in audio input results in an increase in amplification of both units, and additional means associated with said common means for increasing the amplification in the first unit more than in the second unit.

5. In combination, in an audio frequency amplifying system, a source of audio modulated electrical energy, a pair of independent audio frequency electron discharge amplifiers of difierent frequency discrimination characteristics, said source being connected to the input of each of said amplifiers, a control circuit having its input connected to said source, the output of said control circuit being connected to said amplifiers in such a manner that the ratio of the outputs of each amplifier varies in a predetermined manner with the energy level of said source, and an additional arrangement between said control circuit output and one of said amplifiers for varying the energy output thereof at a different rate than the energy output of the other amplifier and in the same direction as the energy level of said source.

6. In combination with a source of audio frequency energy to be amplified, at least two amplifiers, each of which is arranged to stress a predetermined, independent portion of the audio frequency spectrum, means for automatically regulating, in a predetermined manner and in accordance with the energy level of said source,'the tonal output of said amplifiers, and additional means, operative simultaneously with the regulating means, for increasing the total amplification in both amplifiers with an increase of said energy level, but decreasing the total amplification of the amplifiers when the said level becomes abnormally low.

7. In combination with a source of audio frequency modulated electrical currents and reproducing means, an audio amplifier having a high frequency discrimination characteristic, an audio amplifier having a low frequency discrimination characteristic, and a rectifier circuit connected between the source and an amplification control element of each amplifier, said elements being connected to points of such difierent direct current potential of said rectifier circuit that the effective amplification of both amplifiers increases with an increase in intensity of said source currents.

8. In combination with a source of audio frequency modulated electrical currents and reproducing means, an audio amplifier having a high frequency discrimination characteristic, an audio amplifier having a low frequency discrimination characteristic, and a control circuit including an amplifier and a rectifier connected between the source and an amplification control element of each amplifier, said elements being connected to points of such difierent potential of said control rectifier circuit that the efiective amplification of both amplifiers increases with an increase in intensity of said source currents.

9. In combination with a source of audio frequency modulated electrical currents and reproducing means, an audio amplifier having a high frequency discrimination characteristic, an audio amplifier having a low frequency discrimination characteristic, a control circuit including a rectifier, and a potentiometer in the control circuit connected between the rectifier and an amplification control element of each amplifier, said elements being connected to points of different direct current potential of said control circuit potentiometer which are of like polarity.

10. In an amplifying system, means for controlling the audio frequency response characteristic and output volume thereof so that the output increases when the high audio response is increased, and signaL-responsive means for automatically actuating said control means.

11. A wave repeating system comprising means transmitting wave energy more efiiciently at certain frequencies than at others, means transmitting wave energy more efficiently at frequencies other than the said certain frequencies, and means, responsive to the amplitude of a wave impressed on the system, for varying the transmission efficiency of each of said means while simultaneously varying the output of the system in the same direction as changes in said wave amplitude.

12. In combination with an audio amplifier, means automatically actuated in accordance with increasing average audio input intensity for increasing the amplification of the amplifier, and additional means associated with the first means for simultaneous decreasing the ratio of low to high frequency amplification.

WALTER van B. ROBERTS, 

